The role of fluids as agents of mass transfer within terrestrial planets and moons has become a rapidly expanding area of research in Earth and Planetary Sciences. This work builds on very recent advances in thermodynamic modelling which have provided predictive models suggesting that carbon-rich fluids are powerful mass transfer agents. The primary objective of MEDIAN is to challenge the framework within which mantle geochemistry operates and the assumption that diamond inclusion geochemistry reflects the formation environment. MEDIAN will perform a comprehensive and innovative suite of high-pressure, high-temperature multi-layer experiments to constrain the mass transfer power of fluid metasomatism in Earth’s silicate mantle. Additionally, a recalibration of the Deep Earth Water model thermodynamic database will result in an improvement in the accuracy and precision of predictive computational models to predict the outcome of fluid-rock interaction in high-pressure systems. This recalibration will facilitate a comprehensive investigation of environmental conditions that are otherwise challenging to replicate experimentally. In conclusion, MEDIAN will investigate the ability of fluids to precipitate mineral assemblages traditionally associated with magmatic processes, providing new exciting insights into the behaviour of metasomatic fluids and deciphering the important messages trapped in diamond inclusions regarding Earth’s tectonic evolution. MEDIAN will speak to one of the most pressing questions in planetary science: how exactly has the sustained injection of aqueous fluids into the mantle at convergent plate boundaries changed Earth since the onset of plate tectonics?